We need to catch these when they're created. During write is too late.

pcbnew/pcb_io/kicad_sexpr/pcb_io_kicad_sexpr.cpp

@@ -1021,7 +1021,6 @@ void PCB_IO_KICAD_SEXPR::format( const PCB_SHAPE* aShape ) const
         }
         else
         {
-            wxFAIL_MSG( wxT( "Cannot format invalid polygon." ) );
             return;
         }